1. Field of the Invention
The present invention relates to an image processing apparatus, an image processing method, and an image processing program that correct a color conversion table used when reading a document.
2. Description of the Related Art
In recent years, accompanying the downsizing of devices and the improvement of their functions, as a document feeder (DE) to be mounted on a copying machine, a reader has appeared, which is capable of simultaneously reading double surfaces of a document without the need to reverse the document. That is, a reader for simultaneously reading double surfaces comprising two devices of an obverse surface reading device and a reverse surface reading device has appeared. When the reader for simultaneously reading double surfaces is compared with a reader for reading double surfaces by reversal path comprising a reading device only on one side, the former has an advantage such as reduction in reading time and the reduction in the jam occurrence rate because there is only one path for conveying a document.
On the other hand, it has a disadvantage, such as the occurrence of a shift in the reading characteristics between the obverse surface and the reverse surface because the obverse surface and the reverse surface of a document are read by different sensors. The reading characteristics include, for example, color, modulation transfer function (MTF), and color shift, and here, the color is focused on in particular.
In order to match the color of the obverse surface read by a reader for simultaneously reading double surfaces with that of the reverse surface, there can be conceived a method of recreating the respective color conversion tables with precision or a method of correcting a color conversion table of one reading device with that of the other in accordance with the read value thereof. Here, the former method requires a lot of time and effort or expertise to create a color conversion table, and therefore, the latter method, that is, the method of correcting a color conversion table can be conceived as important because any one can do so with ease.
FIG. 11 is a diagram showing a color conversion table format. The color conversion table in FIG. 11 is a table used to convert an L*a*b* value of input data into an L*a*b* value of output data. Here, the table is a through table by which nothing is converted, however, it is possible to set so that a predetermined input data value is converted into a predetermined output data value. It is also possible to perform color space conversion by converting output data into different color space data, such as CMYK.
Conventionally, as a color correction method of a reading device, a lot of methods have been proposed, which effectively correct a color, while being specialized in a color desired to be corrected. In Japanese Patent Laid-Open No. 2008-141625, a method is proposed, in which a background color of a document is extracted automatically and a correction is made while being specialized in the color. In Japanese Patent Laid-Open No. 2005-244519, a method is proposed, in which only a lattice point around a color desired to be corrected is concentrated on and corrected with precision.
However, when the entire color space is corrected, the method disclosed in Japanese Patent Laid-Open No. 2008-141625, in which a correction is made while being specialized in one color, requires data of a large number of colors. Further, the method disclosed in Japanese Patent Laid-Open No. 2005-244519, in which the lattice points around the focused lattice point are condensed, requires a very large number of lattice points after all. This method requires a lot of data and memory size, processing time, etc. It makes no advantage compared to recreating a color conversion table.
In contrast to the above, a method of correcting the entire color space with a small number of patches without the need to recreate entire color conversion table includes a method disclosed in Japanese Patent Laid-Open No. 2005-175806. A method of correcting a destination profile is proposed, in which color conversion is performed from a device-independent color space into a device-dependent color space using a color conversion lookup table, also called an ICC profile, on a side of printer, which is an output device.
However, when the method proposed as above is applied to a color conversion table of a reading device, there arises a serious problem.
FIG. 2A and FIG. 2B are diagrams showing conversions between color spaces. An input device has a color conversion table used to perform conversion from a device-dependent color space (RGB) into a device-independent color space (R′G′B′) as shown in FIG. 2A. On the other hand, an output device has a color conversion table used to perform conversion from a device-independent color space (R′G′B′) into a device-dependent color space (CMYK) as shown in FIG. 2B.
FIG. 3A and FIG. 3B show modeled distributions of lattice point data in color spaces on the input side (FIG. 3A) and the output side (FIG. 3B) in a general color conversion table. On the input side, the lattice points are arrayed at regular intervals, however, the lattice point intervals on the output side are very irregular. This results from a shift in values that occurs when conversion from a device-dependent color space into a device-independent color space or inverse conversion is performed. That is, in order to eliminate this shift, in a general color conversion table, the lattice points on the input side are arranged regularly and the lattice points on the output side are arranged so that the lattice point intervals are irregular, and thereby, an optimum color conversion is realized as a result.
However, when correcting a color conversion table, it is desirable to make a correction in a device-independent color space in which a relationship between the lattice point interval and the color difference is linear. In addition, it is possible to evenly make a correction when the lattice point intervals are regular. When there is a shift in the device characteristics, they do not shift uniformly in a color space, and therefore, a shift to be corrected differs from color region to color region. Consequently, if the lattice point intervals are irregular, it is possible to make a correction with high precision in a part dense with lattice points, however, the correction precision in a part where the lattice points are sparse is deteriorated. With respect to the above-described points, the destination profile of the output device to be corrected in Japanese Patent Laid-Open No. 2005-175806 satisfies the condition that the data on the input side where the lattice point intervals are regular is a device-independent color space. As a result, it is possible to obtain the effect of correction by directly correcting the destination profile. However, correcting a color conversion table of a reading device requires to correct a color conversion table where the data on the input side is a device-dependent color space, and therefore, it is necessary to correct data of lattice points having an uneven distribution as represented in FIG. 3B. Consequently, it is necessary to make a certain effort to directly correct the entire color conversion table without unevenness.
There can be conceived a method of indirectly correcting a color conversion table of a reading device. As a method of indirectly correcting a color conversion table, there is a method that uses a color conversion table for correction as disclosed in Japanese Patent Laid-Open No. 2003-324625. In Japanese Patent Laid-Open No. 2003-324625, the destination profile is not corrected directly to make a correction that has taken total amount of tonner into consideration, but a multidimensional table is created. The configuration is such that the data read by the reading device is corrected using the created multidimensional table and sent to the destination profile. However, in the method disclosed in Japanese Patent Laid-Open No. 2003-324625, a signal value corresponding to a color to be corrected is selected from the reference table and the device is caused to output a color patch. Consequently, a correction table corresponding only to the intended color is created. This method can be said to be optimum to effectively correct a small number of colors, however, in order to correct the entire color space, more patches are required accordingly. Further, it is also necessary to prepare colorimetric values of those patches, and therefore, the number of patches and amount of time and effort increase in accordance with the number of colors to be corrected.
The present invention has been developed in view of the above-mentioned problems and provides a method of evenly correcting a color conversion table of a reading device in the entire color region even with a limited number of patches.